The fair place to meet is Oklahoma City, OK — the city closest to the midpoint of Nashville and San Diego. From the farther side that’s about 23 hr 52 min of driving.
Recommended midpoint
Oklahoma City, OK
From Nashville
12 hr 39 min
603 mi to Oklahoma City
From San Diego
23 hr 52 min
1,138 mi to Oklahoma City
Nashville has the shorter trip; the split is off by about 11 hr 13 min. The alternatives below can even it out. Nashville and San Diego are about 1,740 miles apart.
Nashville and San Diego are about 1,740 miles apart by road. Split the difference and you arrive near Oklahoma City, the city closest to the halfway point between them. That puts roughly 12 hr 39 min of driving on the Nashville side and 23 hr 52 min on the San Diego side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. Oklahoma City still makes a fair, central place for Nashville and San Diego to converge, splitting the travel instead of asking one side to cross the country.
If Oklahoma City doesn't have what you're after, Albuquerque and Dallas are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Nashville and San Diego.
Oklahoma City, often shortened to OKC, is the capital and most populous city in the U.S. state of Oklahoma. It is the 21st-most populous U.S. city and 8th largest in the Southern United States, with a population of 681,054 at the 2020 census. Read more →
Nashville is the capital and most populous city in the U.S. state of Tennessee. It is the seat of Davidson County in Middle Tennessee, located on the Cumberland River. Read more →
San Diego is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. It is the eighth-most populous city in the U.S. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.